98 lines
3.8 KiB
HTML
98 lines
3.8 KiB
HTML
<html>
|
|
<head>
|
|
<!--
|
|
Support idiotic mobile browsers that are incapable of rendering
|
|
straightforward HTML properly
|
|
-->
|
|
<meta name="viewport" content="width=device-width, initial-scale=1">
|
|
<title>Keeping filesystem images sparse</title>
|
|
</head>
|
|
<body>
|
|
<h2>Keeping filesystem images sparse</h2>
|
|
<p>
|
|
Filesystem images in local files can be used by many PC emulators and
|
|
virtual machines
|
|
(<a href="http://user-mode-linux.sourceforge.net/">user-mode Linux</a>,
|
|
<a href="http://fabrice.bellard.free.fr/qemu/">QEMU</a> and
|
|
<a href="http://wiki.xensource.com/xenwiki/">Xen</a>, to name but three).
|
|
Typically these filesystems are created as sparse files using
|
|
commands like:
|
|
<p>
|
|
<pre>
|
|
dd if=/dev/zero of=fs.image bs=1024 seek=2000000 count=0
|
|
/sbin/mke2fs fs.image
|
|
</pre>
|
|
where the enormous <code>seek</code> value causes <code>dd</code> to move
|
|
forward by 2GB before writing nothing at all. This results in the
|
|
creation of a sparse file which takes disk space only for blocks which are
|
|
actually used:
|
|
<p>
|
|
<pre>
|
|
$ ls -l fs.image
|
|
-rw-rw-r-- 1 rmy rmy 2048001024 Apr 18 19:10 fs.image
|
|
$ du -s fs.image
|
|
31692 fs.image
|
|
</pre>
|
|
As the filesystem is used, more and more of the non-existent blocks are
|
|
filled with data and the size of the file on disk grows. Sometimes it would
|
|
be nice to be able to reclaim unused blocks from a filesystem image. However,
|
|
deleting files from the image doesn't return the space to the
|
|
underlying filesystem: even free blocks in the image still consume space.
|
|
Reclaiming the space can be achieved in two stages:
|
|
<ul>
|
|
<li>Fill unused blocks with zeroes
|
|
<li>Make the file sparse again
|
|
</ul>
|
|
<p>
|
|
One traditional way to zero unused blocks is to create a file that fills
|
|
all the free space:
|
|
<p>
|
|
<pre>
|
|
dd if=/dev/zero of=junk
|
|
sync
|
|
rm junk
|
|
</pre>
|
|
<p>
|
|
The disadvantage of <code>dd</code> in this context is that it destroys
|
|
any sparseness that exists: free blocks that were originally represented
|
|
as holes in the image file are replaced with actual blocks containing
|
|
zeroes. Also, filling up a live filesystem is probably a bad idea.
|
|
<p>
|
|
As an alternative approach, and as practice in mucking about with ext2
|
|
filesystems, I've written a utility which scans the free blocks in an
|
|
ext2 filesystem and fills any non-zero blocks with zeroes.
|
|
The source, <a href="zerofree-1.1.1.tgz">zerofree-1.1.1.tgz</a>, is
|
|
available for download.
|
|
<p>
|
|
<ul>
|
|
<li>A cautious user would run fsck on the filesytem both before and after
|
|
running zerofree.
|
|
<li>The filesystem to be processed should be unmounted or mounted read-only.
|
|
<li>The utility also works on ext3 or ext4 filesystems.
|
|
<li>Binary packages are available in the standard repositories for Debian and Fedora.
|
|
<li>The <a href="http://www.sysresccd.org/SystemRescueCd_Homepage">SystemRescueCd</a> live distribution includes zerofree.
|
|
<li><a href="http://libguestfs.org/">guestfish</a> can run zerofree on many types of virtual machine filesystems.
|
|
</ul>
|
|
<p>
|
|
However, this is only half the story: the empty free
|
|
blocks still consume space in the underlying filesystem, so something
|
|
must to be done to reclaim that space.
|
|
<p>
|
|
A common suggestion is to use the sparse file handling capabilities
|
|
of the GNU <code>cp</code> command to take a copy of the filesystem image with
|
|
<code>cp --sparse=always</code> (though this does require the original
|
|
and sparse files to exist at the same time, which may be inconvenient).
|
|
<p>
|
|
If your kernel and util-linux are sufficiently modern and you have a supported
|
|
filesystem you can use <code>fallocate -d</code> to 'dig holes' in a file.
|
|
This makes the file sparse in-place, without using extra disk space.
|
|
<p>
|
|
<hr>
|
|
<address>
|
|
<a href="mailto:rmy@pobox.com">Ron Yorston</a><br>
|
|
18th April 2004 (updated 19th February 2018)<br>
|
|
Some <a href="obsolete.html">obsolete</a> information has been moved to a
|
|
separate page.
|
|
</address>
|
|
</body>
|
|
</html>
|